Most hydraulic jars are capable of jarring up, but not down. It is difficult to jar in both directions using a single hydraulic valve, due to problems in getting the valve to center itself properly in the restriction. For that reason most two way hydraulic jars, use two hydraulic valves, one of which is inverted.
U.S. Pat. No. 5,123,493 is an example of a two way hydraulic drilling jar that utilizes a single valve. U.S. Pat. No. 5,123,493 discloses an hydraulic drilling jar which is constructed of a mandrel that is telescopically received within a tubular housing. An annular fluid chamber is formed between the mandrel and the housing. An annular valve member is disposed within the fluid chamber and is secured between two projecting shoulders on the exterior surface of the mandrel. As relative movement of the mandrel and the housing occurs the valve member moves axially in the fluid chamber. The interior surface of the housing has a restriction. The valve member moves freely in the fluid chamber until it reaches the restriction. Bypass passages are provided to enable fluid to bleed past the valve member when the valve member is positioned within the restriction. Should the drill string become stuck, the drill string is placed in either compression or tension, depending upon whether it is desired to jar up or jar down. The force exerted upon the drill string tends to cause relative movement of the mandrel and the housing. Only limited relative movement can occur, however, until after a time delay during which sufficient fluid bleeds through the bypass passages to enable the valve member to come out of the restriction. Once the valve member comes out of the restriction, the mandrel and housing are free to move and a hammer on one is brought into engagement with an anvil on the other in a violent jarring impact.